House flies (Musca domestica) are the probable carriers of more than 65 diseases and a major threat to human health. Insecticides are enormously important for fly control. However, a major problem with the use of insecticides is the development of resistance in this pest. Pyrethroid insecticides are currently widely used for fly control and have many beneficial properties including low human toxicity and low environmental disruption compared to many previously used compounds. An important mechanism by which house flies become resistant to pyrethroid insecticides is through the increased oxidative metabolism mediated by the cytochrome P450 microsomal monooxygenases. The microsomal monooxygenases are an extremely important metabolic system capable of metabolizing a wide range of compounds due to the presence of several different cytochrome P450 isoforms. Our understanding of monooxygenase-mediated insecticide resistance has been limited because of the difficulty in isolating the individual P450s involved. Cytochrome P450/lpr is a house fly specific P450 that is responsible for the high level of monooxygenase mediated pyrethroid resistance in the LPR strain of house fly. P450/lpr is coded for by the CYP6D1 gene which we have recently sequenced from LPR and susceptible house flies. In order to understand the molecular basis of CYP6D1-mediated resistance, two specific aims will be addressed: 1) identify CYP6D1 regulatory elements involved in resistance and 2) evaluate the catalytic activity of CYP6D1 protein coded for by the different CYP6D1 alleles from LPR and susceptible house flies toward pyrethroid insecticides. The proposed research will give new insights into the processes by which an insect is able to become resistant to insecticides by the expression of increased levels of monooxygenase activity. The ultimate goal of this research is to find ways to circumvent the development of monooxygenase- mediated pyrethroid resistance.